The present invention pertains to propulsion transmissions for large ships which are capable of operating at high speeds. Ships of this character utilize large prime movers or power sources, such as gas turbine engines or diesel engines for example, and conventionally use controllable pitch propellers as a means for reversing the direction of the ship. These transmissions utilize marine reduction gear means that have no capability of reversing the direction of propeller rotation. Serious problems are encountered with controllable pitch propellers when used in such transmissions because the propellers often fail, have high maintenance, and require dry-docking of the ship and consequent periods of inoperativeness.
In ships of the character to which the present invention pertains, it is desirable to be able to reverse the direction of the ship, sometimes in "crash reversal" conditions, and to do so requires that the transmission must bring the forwardly rotating ship propeller to a stop and then reverse it even while the ship itself continues forward at appreciable speed. These "crash reversals" cause this transient condition of propeller reversal to be undertaken at very high forward ship velocities so that minimum "reach" (continued forward travel of the ship) is possible.
When the prior art transmissions have been utilized under such conditions, the reversal of the propeller causes severe forces to be transmitted back to the prime mover and other interconnecting components, such as clutches. Among some prior art arrangements was the use of a friction clutch for both forward and reverse operation. In order to protect the prime mover, such as a gas turbine, these clutches had to provide a synchronized lock-up and no slip drive at full engine power ahead or astern, had to provide modulation of low propeller speeds, and prime mover speeds of less than full throttle necessitated that the clutch would slip continuously without thermal damage and with excellent propeller speed control at propeller speeds of one-half normal down to the minimum speed expected. Furthermore, the same clutches had to be of a fixed size and of a thermal capacity to reverse the propeller from its former direction to that required, notwithstanding the severe propeller and ship hull dynamics involved.
In transmissions of the above character, there are a number of limiting parameters, one of which is the hull speed at which it is necessary to reverse the propeller direction. This particular parameter directly effects the distance that the ship will travel before actually stopping (reach).
Solutions of this transient reversal problem include a number of sequential steps to finally achieve reverse propeller rotation. Some of these consist of a variety of devices that first slow down the propeller, discretely stop and hold the propeller in a stalled condition, and then initiate propeller rotation in the reverse direction and finally achieve steady-state reverse operation.
Other prior attempts have been made to provide suitable transmissions for reversing large ships. An example of one such transmission is shown in the U.S. Pat. to Lysholm No. 2,140,324 issued Dec. 13, 1938 which utilized a three stage torque converter that drove a hydraulic coupling which could be selectively braked to reverse the power direction. This arrangement however made the reversal assume the characteristics of the simple hydraulic coupling and consequently, when the ship prime mover interaction made the coupling reach a negative speed ratio, it was impossible to get the necessary high output torque and instead the output torque decayed rapidly when operating in the reverse direction.
Another example of a prior art transmission of this general character is shown in the British Pat. No. 547,330 of 1942 wherein simple fluid couplings were used, but which were also incapable of torque amplification in reverse. This transmission also resulted in the output torque collapsing completely and quickly in the negative speed ratio zone of near minus (-) 1.0.
Neither of the two prior art transmissions could provide any torque amplification in the negative speed zones. If the coupling were sized to provide large torques in the negative speed ratio operational zone, then these torques would be reflected back into the prime mover.